Fluid catalytic cracking (FCC) processes and the related plants wherein hydrocarbon feedstocks are catalytically cracked are well known in the art. In FCC processes a preheated hydrocarbonaceous feedstock of a high boiling point range is brought into contact with a hot cracking catalyst in a riser. The feed is cracked into lower boiling products, such as gas, LPG, gasoline, and cycle oils. Furthermore, coke and non-volatile products deposit on the catalyst resulting in a spent catalyst. The riser exits into a separator wherein the spent catalyst is separated from the reaction products. In the next step the spent catalyst is stripped with steam to remove the non-volatile hydrocarbon products from the catalyst. The stripped catalyst is passed to a regenerator in which coke and remaining hydrocarbonaceous materials are combusted and wherein the catalyst is heated to a temperature required for the cracking reactions. Hereafter the hot regenerated catalyst is returned to the riser reactor zone.
FCC regenerators are generally equipped with first and second stage cyclones. These are normally mounted inside the regenerator vessel. In these systems the outlet duct of the first stage cyclone is coupled directly to the inlet duct of the second stage cyclone. An example is given in “Fluid Catalytic Cracking: Technology and Operation”, Joseph W. Wilson, PennWell Publishing company, 1997, ISBN 0-87814-710-1, page 183-185. The cyclone separation step results in a gas rich overflow and a solids rich underflow. The solids of the underflow are directed back to the regenerator vessel.
The overflows of these separators are usually collected in a gas collection chamber, and are called regenerator flue gases. The regenerator flue gas still contains fine catalyst particles. From an environmental standpoint it is undesired to discharge this gas untreated. Therefore, third stage separators (TSS) have been utilized for many years to separate catalyst fines from the regenerator flue gas. Several designs are available. The most widely used design is the Shell separator, which was developed by Shell to protect turboexpanders from catalyst particles in the flue gas. The separator consists of a vessel which contains numerous swirl tube separators. These separators are small axial flow cyclones. Flue gas entering the separator tube passes through the swirl vanes which imparts a spinning motion to the gas flow. The resulting forces move the catalyst particles to the tube wall where they are separated from the gas stream. These swirl tube separators and are for example described in “Fluid Catalytic Cracking: Technology and Operation”, Joseph W. Wilson, PennWell Publishing company, 1997, ISBN 0-87814-710-1, page 168-170. The separated particles fall through the bottom of the tubes and are collected in the conical bottom of the separator vessel. The separated particles are discharged from the vessel together with a small quantity of the flue gas. This particles-rich flow is also referred to as the TSS underflow. This TSS underflow is then routed to an underflow separator, or a so-called fourth stage separator (FSS). The TSS overflow is routed to the stack.
Although the TSS has been used as an effective device to remove catalyst fines from the regenerator flue gas, emission to stack is also largely influenced by the catalyst loss from the FSS. In the FSS particles are for instance separated by a so-called 4th stage cyclone. The separation results in a gas overflow that is directed to the stack and particles that are generally removed as waste material.
The basic mechanism for particle separation in gas cyclones is that due to circular motion a centrifugal force pulls the particle against the wall, and at the wall the boundary layer carries the separated particle to the dust outlet. In the case of small particles the centrifugal force may be smaller than the drag force of the particles and thus the separation of these particles can be difficult. However, these small particles are often found in the coarse fraction, even when cyclones are not suitable for separating such small particles. Studies have revealed that some small particles may form larger aggregates, or agglomerates, where particles are interlinked in a stable formation. The small particles in the aggregates stick together due to inter-particles forces, e.g. van der Waals forces, electrostatic forces and capillary forces (see for example “Gas Cyclones and Swirl Tubes: principles, design and operation”, A. C. Hoffmann and L. E. Stein, Springer, 2002, ISBN 3-540-43326-0 and S. Obermair et al, Powder Technology 156 (2005) 34-42). To optimise the separation of such small particles it is desirable to retain the agglomerates from one separator when they are fed to a further separator. This may be the case in transporting the TSS underflow to the FSS.